In a chain transmission device for driving camshafts, the device including a crank sprocket attached to a crankshaft, cam sprockets attached to the respective camshafts, and a timing chain (hereinafter, simply referred to as “chain”) having a loose side chain portion and being trained around the crank sprocket and the cam sprockets. A pivotable chain guide extending along the direction in which the chain moves is provided on one side of the loose side chain portion, and an adjustment force is applied to an end portion of the chain guide by a chain tensioner such that the chain becomes tensioned, thereby preventing the chain from becoming loose or flapping.
Also, a fixed chain guide is mounted to the tension side chain portion of the chain with both end portions of the fixed chain guide supported by respective supporting members. The fixed chain guide guides the movement of the chain, while preventing the chain from flapping.
As the chain guide for adjusting the tension of the chain or the chain guide for guiding the movement of the chain, a sliding-type chain guide is known which guides a chain by its convex circular-arc shaped guide surface. However, since such a sliding-type chain guide guides a chain by coming into sliding contact with the chain, there is the problem that the resistance to the movement of the chain is large so that the transmission loss of torque is large.
In order to overcome such a problem, the applicant of the present application has proposed, in PCT International Publication No. 2010/090139, a chain guide including a guide base made of synthetic resin, and elongated in the direction in which a chain moves, a plurality of roller shafts arranged so as to draw a curved line, and each having both end portions supported by the guide base, and rollers comprising roller bearings. The rollers are rotatably supported by the respective roller shafts, and the rollers movably guide the chain.
Since the rollers roll to guide the chain in the above chain guide, the resistance to the movement of the chain is small so that the transmission loss of torque is small.
In the chain guide of the above-noted PCT International Publication No. 2010/090139, the chain linearly moves between adjacent rollers, but moves while curving at the positions of the chain coming into contact with the respective rollers so as to collide against the outer peripheries of the rollers. Therefore, large noise or vibration is likely to occur compared to a conventional chain guide slidably guiding a chain by its circular-arc shaped guide surface. Therefore, it is necessary to reduce such noise or vibration.
In a chain transmission device for driving camshafts, the device includes a crank sprocket, a chain, and rollers. When the torque of the crank sprocket changes due to the explosion of an engine, the tension of the chain changes largely. At this time, the roller located closest to the crank sprocket is likely to be affected by the flapping of the chain, so that noise is likely to occur especially at this roller.
In order to learn the relationship between a noise and the winding angles α of the portions of a chain C located on the outer peripheries of rollers R comprising roller bearings and guiding the chain C by rolling as illustrated in FIG. 4, by varying the above winding angles α, the inventors of the present application performed an experiment on the noise generated when the chain C moves at a constant speed. As a result of this experiment, the inventors obtained the experimental result illustrated in FIG. 5. From the graph of FIG. 5, it turned out that a sound pressure value decreases by setting the winding angles α of the chain C to be large, but does not decrease any further if the winding angles α exceed 170 degrees.
The inventors performed a further experiment so as to measure noise under the condition that the winding angle α of the portion of the chain C located on the outer periphery of the roller R located closest to a crank sprocket is 170 degrees, and discovered that it is possible to markedly reduce noise under this condition.
The winding angle of the chain C means the intersecting angle of the common tangent of one adjacent pair of any three rollers R arranged successively in the direction in which the chain C moves, and the common tangent of the other adjacent pair of the three rollers R (see FIG. 4), or the intersecting angle of the common tangent of the crank sprocket and the roller R adjacent to the crank sprocket on its chain exit side and the common tangent of this roller R and the roller R adjacent to this roller R (see FIG. 2).
In a chain transmission device for driving camshafts in which a plurality of guide rollers provided in a chain guide guides a chain by rolling, it is an object of the present invention to reduce the noise generated when the chain moves.